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Vikram Jeet,

Thanks for responding. I understand the first 2 terms of your equation. But I am not clear about the 3rd and 4th terms.

Term 1 : L * B * T * Vc represents the weight of the footing concrete, where L, B and T are the length, width and thickness of the footing, and Vc is the unit weight of concrete. Seems OK.

Term 2 : L * B * Vd * Dd represents the weight of dry soil which is above the wet soil.  Seems OK.

Term 3 : L * B * Vs * Dw represents what? The weight of saturated soil assuming it occupies the entire space from base of footing to bottom of dry soil?

Term 4 : L * B * Vw * Dw represents what? The weight of water contained in the zone between base of footing and bottom of dry soil?

When you combine terms 3 and 4, are you saying that the space between bottom of dry soil and bottom of footing is occupied entirely by submerged soil (which is what (Vs-Vw) represents)? If so, aren't you double counting the loads in the zone occupied by the footing?

Back to my original question? What is the buoyant force on the footing?

[vikram.jeet]

Weight of soil  and water on Footing =
Dry zone soil wt based on dry density
Plus
Soil weight based on submerged density generally taken as  (Vs-Vw) * in submerged zone
Plus
Weight of water over footing in submerged zone

Net weight is Buoyant weight = Sum Above - Uplift  Force

If submerged unit weight is available in report it can be taken directly ,,  but very nearly it can be assumed as ( Vsat - Vw)

[vikram.jeet]

Alternatively

Buoyant weight = L*B*T( Vc - Vw)   +  LB Dw * Vsub   + LB*Dd * Vdry
( In this case Uplift force ( upwards is already considered and shall not figure separately)

But It is always better to consider Gross weights  of items above Footing including it's self wt. And co-existing Uplift pressure diagram at base line. This helps in working out Net moments and shears  properly.

Gross weight = Dry soil wt  + Submerged soil wt based on sub. Density + Wt of water on footing of submerged zone + weight of footing

Net weight =  Byount wt = Gross W  minus Uplift  pr force

[vikram.jeet]

MODIFIED DT 25.03.2025 TO CLARIFY


UPLIFT PRESSURE

Best way is to take as under :
Df = Depth of Foundation
Dw = The Ht of water level above Footing base submerged soil zone
Dd  = ( Df - Dw)  taking as  dry soil zone

Uplift pressure U = Vw *Dw
Vw= is unit wt of water = 1000 kg/m3
Vs = Saturated soil unit weight

Vd = Dry unit weight of soil above water table
A= plan area of footing

Wt of soil & Footing Minus Uplift = ( A*T * Vc )+ (A*Vd*Dd) + { A*Vs*(Dw-T) }  -   (A*Vw*Dw)

IDEAS IS
TAKE FULL WEIGHTS OF ALL COMPONENTS AND DEDUCT UPLIFT PRESSURE FORCE. IE TAKE SATURATED WEIGHT OF SOIL UPTO WATER TABLE PLUS DRY WT. ABOVE IT MINUS UPLIFT FORCE
THIS WILL AMMOUNT TO TAKING BUYONT WT OF COMPONENTS SUBMERGED

Modified to clarify dated 25.03.2025